Soft Superconducting Gap in Semiconductor Majorana Nanowires

Takei, So; Fregoso, Benjamin M.; Hui, Hoi-Yin; Lobos, Alejandro M.; Sarma, S. Das

doi:10.1103/physrevlett.110.186803

Cited by 144 publications

(167 citation statements)

References 55 publications

(99 reference statements)

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“…The dV/dI (V SD ) value is reduced in the gap region and a U-shape dip which is a signature of a hard gap [5] is observed for junction numbers 2, 3, 5, and 7, where supercurrents with critical currents of up to 2 µA are measured. However, the results for junction numbers 1, 4, and 6 are different and they show a soft gap signature [9,10] in which the junction resistance drops but dV/dI (V SD ) is not flat. The origin of the soft gap is not fully understood, however, this has so far been explained by the effect of inhomogeneities at the S-Sm interfaces together with quasiparticle broadening.…”

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confidence: 96%

“…The origin of the soft gap is not fully understood, however, this has so far been explained by the effect of inhomogeneities at the S-Sm interfaces together with quasiparticle broadening. [9] In our junctions, this difference may be due to a residue altering the wet etching process resulting in a tunneling barrier at the interface or variation in the etch depth so that the Nb trench in which the Nb sits is not deep enough in some areas. In this case, the resulting tunneling barrier leads to a reduction of the excess current with both normal and Andreev reflections competing together.…”

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confidence: 99%

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On‐Chip Andreev Devices: Hard Superconducting Gap and Quantum Transport in Ballistic Nb–In_0.75Ga_0.25As‐Quantum‐Well–Nb Josephson Junctions

Delfanazari

Puddy

et al. 2017

Advanced Materials

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A superconducting hard gap in hybrid superconductor–semiconductor devices has been found to be necessary to access topological superconductivity that hosts Majorana modes (non‐Abelian excitation). This requires the formation of homogeneous and barrier‐free interfaces between the superconductor and semiconductor. Here, a new platform is reported for topological superconductivity based on hybrid Nb–In0.75Ga0.25As‐quantum‐well–Nb that results in hard superconducting gap detection in symmetric, planar, and ballistic Josephson junctions. It is shown that with careful etching, sputtered Nb films can make high‐quality and transparent contacts to the In0.75Ga0.25As quantum well, and the differential resistance and critical current measurements of these devices are discussed as a function of temperature and magnetic field. It is demonstrated that proximity‐induced superconductivity in the In0.75Ga0.25As‐quantum‐well 2D electron gas results in the detection of a hard gap in four out of seven junctions on a chip with critical current values of up to 0.2 µA and transmission probabilities of >0.96. The results, together with the large g‐factor and Rashba spin–orbit coupling in In0.75Ga0.25As quantum wells, which indeed can be tuned by the indium composition, suggest that the Nb–In0.75Ga0.25As–Nb system can be an excellent candidate to achieve topological phase and to realize hybrid topological superconducting devices.

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confidence: 96%

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confidence: 99%

On‐Chip Andreev Devices: Hard Superconducting Gap and Quantum Transport in Ballistic Nb–In_0.75Ga_0.25As‐Quantum‐Well–Nb Josephson Junctions

Delfanazari

Puddy

et al. 2017

Advanced Materials

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“…4-7 However, preliminary experiments probing Majorana bound states in nanowires were marked by disorder and a soft superconducting gap in the tunneling regime. [4][5][6][7][8] Disorder in nanowire systems is known to break up ballistic transport 9,10 , which is a crucial ingredient for developing 1D topological superconductivity. 1, 2, 4, 9 Additionally, disorder can produce zero-bias conductance signatures similar to Majorana bound states.…”

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confidence: 99%

Hybrid superconductor-quantum point contact devices using InSb nanowires

Gill

Damasco

Car

et al. 2016

Applied Physics Letters

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Proposals for studying topological superconductivity and Majorana bound states in nanowires proximity coupled to superconductors require that transport in the nanowire is ballistic. Previous work on hybrid nanowire-superconductor systems has shown evidence for Majorana bound states, but these experiments were also marked by disorder, which disrupts ballistic transport. In this letter, we demonstrate ballistic transport in InSb nanowires interfaced directly with superconducting Al by observing quantized conductance at zero-magnetic field. Additionally, we demonstrate that the nanowire is proximity coupled to the superconducting contacts by observing Andreev reflection. These results are important steps for robustly establishing topological superconductivity in InSb nanowires. MainInAs and InSb nanowires (NWs) coupled to superconductors are promising material candidates for studying topological superconductivity harboring Majorana bound states 1,2 and demonstrating non-Abelian particle statistics relevant for topological quantum computation. 3 The basic procedure to observe Majorana zero modes involves tuning a superconducting proximity coupled quantum wire (i.e. a ballistic 1D system) with strong spin-orbit coupling and one spin degenerate mode in magnetic field. 1 Indeed, evidence for Majorana bound states has been observed in proximity-coupled InSb and InAs NWs as a zero-bias conduction peak in tunneling experiments. 4-7 However, preliminary experiments probing Majorana bound states in nanowires were marked by disorder and a soft superconducting gap in the tunneling regime. [4][5][6][7][8] Disorder in nanowire systems is known to break up ballistic transport 9,10 , which is a crucial ingredient for developing 1D topological superconductivity. 1, 2, 4, 9 Additionally, disorder can produce zero-bias conductance signatures similar to Majorana bound states. 11 While the typical signature of ballistic 1D transport-quantized conductance-has been observed in InSb nanowires at high magnetic field 9 and more recently at zero-field, 19 clear demonstrations of ballistic transport at lower fields (< 1T, i.e., before the expected onset of topological superconductivity) in hybrid nanowire-superconductor systems are lacking. Hence, in order to clearly demonstrate topological superconductivity and remove alternative 2 mechanisms for observing zero bias conduction peaks, quantized conductance should be observed at zero magnetic field in NWs proximity coupled to superconductors.Quantized conductance at zero-magnetic field is the step-like increase of conductance with gate voltage through a ballistic 1D constriction, in units of the quantum of conductance, G 0 = 2e 2 /h, for each spin degenerate subband. 12 While quantized conductance in QPCs defined on 2D electron gas (2DEG) materials, such as GaAs heterostructures, is well established, 12-14 demonstrations of quantized conductance in nanowire systems are sparse. In contrast to QPCs in 2DEGs, short nanowire channels contacted by metals are more prone to backscattering ef...

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“…Note Added: After the original submission of this manuscript, a number of theoretical papers [27][28][29][30][31][32][33][34][35] appeared addressing various aspects of the experimental work on the possible observation of the Majorana zero mode in semiconductor-superconductor hybrid structures.…”

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confidence: 99%

To Close or Not to Close: The Fate of the Superconducting Gap Across the Topological Quantum Phase Transition in Majorana-Carrying Semiconductor Nanowires

Stãnescu

Tewari

et al. 2012

Phys. Rev. Lett.

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We investigate theoretically the low-energy physics of semiconductor Majorana wires in the vicinity of a magnetic field-driven topological quantum phase transition (TQPT). The local density of states at the end of the wire, which is directly related to the differential conductance in the limit of point-contact tunneling, is calculated numerically.We find that the dependence of the end-of-wire local density of states on the magnetic field is nonuniversal and that the signatures associated with the closing of the superconducting gap at the Majorana TQPT are essentially invisible within a significant range of experimentally relevant parameters. Our results provide a possible explanation for the recent observation of the apparent nonclosure of the gap at the Majorana TQPT in semiconductor nanowires.PACS numbers: 03.67. Lx, 03.65.Vf, 71.10.Pm The recently reported observation [1] of a zero bias peak (ZBP) in conductance measurements on semiconductor (SM) nanowires coupled to superconductors (SCs) may represent the first experimental evidence of Majorana fermions (MFs), which are theoretically predicted to exist in topological superconductors [2]. Topological SC states capable of supporting MFs can be realized in SM wires with proximity-induced superconductivity by driving the system through a topological quantum phase transition (TQPT) using a suitably directed magnetic field [3][4][5]. At the TQPT, the SC gap necessarily vanishes [2][3][4][5][6]. The absence of any signature associated with the gap closure casts serious doubt on the Majorana fermion interpretation of the ZBP in the recent experiment [1].In this Letter, we offer a possible explanation for the observed non-closure of the gap. By solving numerically an effective tight-binding model for multiband nanowires with realistic parameters we show that, in the vicinity of the TQPT, the amplitude of the low-energy states near the ends of the wire may be orders of magnitude smaller than the amplitudes of the localized MFs. Consequently, the contributions of these states to the end-of-wire tunneling conductance and LDOS are essentially invisible, which results in an apparent non-closure of the gap in these quantities. By contrast, the closing of the gap mandated by the TQPT is clearly revealed by other quantities, such as the total density of states (DOS) and the LDOS near the middle of the wire. We also show that this nonclosure of the gap is non-universal, being dependent on the behavior of certain low-energy wave functions, and we identify the parameter regimes in which signatures associated with the closing of the gap are present in the end-of-wire LDOS.In Ref.[2] Sau et al. proposed that a spin-orbit (SO) coupled semiconductor (SM) thin film with Zeeman spin splitting and proximity induced s-wave superconductivity could be used to realize Majorana fermions. For small Zeeman splitting Γ, the semiconductor is in a conventional (proximity-induced) superconducting state with no MFs, while for Γ larger than a critical value Γ c (corresponding to the TQPT where ...

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Soft Superconducting Gap in Semiconductor Majorana Nanowires

Cited by 144 publications

References 55 publications

On‐Chip Andreev Devices: Hard Superconducting Gap and Quantum Transport in Ballistic Nb–In_0.75Ga_0.25As‐Quantum‐Well–Nb Josephson Junctions

On‐Chip Andreev Devices: Hard Superconducting Gap and Quantum Transport in Ballistic Nb–In_0.75Ga_0.25As‐Quantum‐Well–Nb Josephson Junctions

Hybrid superconductor-quantum point contact devices using InSb nanowires

To Close or Not to Close: The Fate of the Superconducting Gap Across the Topological Quantum Phase Transition in Majorana-Carrying Semiconductor Nanowires

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Soft Superconducting Gap in Semiconductor Majorana Nanowires

Cited by 144 publications

References 55 publications

On‐Chip Andreev Devices: Hard Superconducting Gap and Quantum Transport in Ballistic Nb–In0.75Ga0.25As‐Quantum‐Well–Nb Josephson Junctions

On‐Chip Andreev Devices: Hard Superconducting Gap and Quantum Transport in Ballistic Nb–In0.75Ga0.25As‐Quantum‐Well–Nb Josephson Junctions

Hybrid superconductor-quantum point contact devices using InSb nanowires

To Close or Not to Close: The Fate of the Superconducting Gap Across the Topological Quantum Phase Transition in Majorana-Carrying Semiconductor Nanowires

Contact Info

Product

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On‐Chip Andreev Devices: Hard Superconducting Gap and Quantum Transport in Ballistic Nb–In_0.75Ga_0.25As‐Quantum‐Well–Nb Josephson Junctions

On‐Chip Andreev Devices: Hard Superconducting Gap and Quantum Transport in Ballistic Nb–In_0.75Ga_0.25As‐Quantum‐Well–Nb Josephson Junctions